Art of reducing visibility



Oct. 10, 1944.

REFLEC TA/VCE (PER cE/wj 6 s. CABOT 2,359,746

ART OF REDUCING VISIBILITY Filed Jan. 12, 1943 10 5 a Sr 400 500 600 700 800 WAVE LENGTH IN mu mu INVENTOR. SAMUEL Cnaor ATTORNEYS 600 WAVE LENGTH /N mu mu Patented Oct. 10, 1944 UNITED STATES PATENT osrics 2,359,746 ART or asnucmd vrsmmrrr Samuel Cabot, Boston, Mass. Application January 12, 1943, Serial No. 472,131

9 Claims.

My present invention provides for a paint, enamel or lacquer type of compound to be applied on the surface of objects to reduce their visibility when seen at a distance against the sky or water or a background a considerable distance beyond the object. Such'a paint, enamel or lacquer is suitable to treat military airplanes, ships or land installations which it is important to keep from being observed by the enemy.

The theory on which my paint, enamel or lacquer is compounded involves the duplication artificially on surfaces of conditions in nature producing the scattering of light predominantly of short wave length, which causes the blue of sky, water or dark objects seen at a distance. This blue eifect caused by particles in the atmosphere, smaller than the wave length of visible light, when struck by light, usually that from the sun. The result produces a scattering of the shorter, more easily refracted or reflected rays and the persistence or penetration of the longer ones. The shorter, bluer waves return in part at least to the eye and the longer red and orange stellar space.

According to my invention I produce a similar condition and appearance artificially in a surface coating by combining a coarse black, blackish or dark pigment, which functions in the same way as the black of interstellar space, and a white, whitish or light shade of pigment which is opaque to the short waves of visible light, the particle size of which is smaller in diameter than such wave lengths. These fine pigment particles are dispersed colloidally in the paint, enamel or lacquer vehicle; the coarse black or dark particles are not colloidally dispersed but are merely suspended as they are above colloidal dimensions.

When the compound so made is applied to a surface it will be found that a continuous phase of the highly dispersed particles will completely surround an intermittent phase of the black, or dark coarse particles. White light striking this fllm will have its shorter waves returned as a bluish effect while the longer ones will be absorbed by the coarse, black, or dark particles and turned into heat. This bluish light so reones are lost in the black of interflected is what is known to physicists as the The coarse, black, blackish or otherwise dark particles are not required to be actually coarse as unit particles, but may instead be aggregates of small particles. They may also be small black or dark particles surrounding and adhering to or adsorbed on a coarse particle or aggregate of some inert filler or extender, such as talc or silica as a nucleus.

The underlying physical phenomena and the theory involved have been largely discussed in my prior pending application Serial No. 405,384, filed August 4, 1941.

In that application, as one instance, I described the treatment of an airplane surface by painting it black, if it did not happen to be sufflciently dark in its then color to absorb most of the light rays that might fall upon it. Over that I formed a coat by a paint or lacquer containing uncolored particles of colloidal fineness. As pointed out, I found that white, which really is not a color, gave very excellent results which could be varied by varying the thickness .of the overlying film or the degree of concentration of the colloidal particles in the vehicle. These variations provided surfaces that respond to incident solar light as it happened to be at any particular moment, i. e. under changing conditions of atmospheric conditions.

In my present invention I have conceived of a coating in which the colloidal particles are in a continuous phase and surround black or substantially black particles which are highly opaque. whereby in a single layer or coating I attain the eflect which is described as the Tyndall effect and in which the black or highly absorptive particles quench substantially all of the light waves falling on them. These colloidal particles'are preferably of a dimension between 10 and 200 mu mu. The black particles or aggregates may be of almost any size up to that of reasonable suspension in. the dispersion medium used. This is because it is important that the light waves of length up to and including the blue-green portion of the spectrum be reflected from the un'-- colored particles, so that the light of greater wave lengths pass between them and striking the black or dark absorbent particles are thereby quenched, as is light entering the darkness of interstellar space.

The light reflected from the colloidal particles to which they are opaque appears as blue or gray in daylight depending on weather conditions. On a bright sunny day when there is much blue from the sunlight, the reflection from the above described particles will be predominantly blue. When the sky is overcast and little of the blue light from the sun can penetrate the clouds, the particles will tend to be gray.

As in my previous application, it will be understood that while my present invention is of particular importance for airplane surfaces, it

reduce the visibility of the runways and hangars of a flying field or other buildings which might be the object of enemy attack.

The depth of tone of the blue effect of this coating will. depend on the concentration and the opaqueness to blue light or the colloidal particles in relation to the blackor dark coarse particles. It the white colloidal continuous phase surrounding the black is highly concentrated, or opaque, the color will be light or less blue as is the case of the sky at. the horizon and for the same reason, namely, that there are more particles in the atmosphere between the eye and black interstellar space at the horizon than at the zenith. A sky at the horizon may often have say 30% reflectance while at the zenith it may be 13%. These are, of course, merely illustrative figures as the sky may have wide degrees of variance in its reflectance.

This matter of reflectance is important as it enables those responsible for camouflage to ascertain the degree to which the light at any particular period of time or place is reflected from the sky. This is generally based on an arbitrary percent scale running from which would be practically no light, to 100 which would be theoretically the total reflection possible.

In referring to camouflage, it will be understood that my invention differs from the ordinary basis of camouflage, which is usually efiected by broken lines, areas or colors, whereas my invention provides for a surface condition on a plane wing or body whereby at increasing altitudes the parts lose form and eventually, according to atmospheric conditions at certain altitudes, say 6000 feet, become difllcult to see or often disappear entirely. The only portions, under those conditions, really visible are those in heavy shadow which, of course, constantly change and cannot be anticipated or counteracted. Where atmospheric effect is required over backgrounds of green for land, red for clifis, etc., the undercoat may be of these colors instead of black.

While my invention obviously involves difllculties in illustration, I have endeavored in the accompanying drawing to suggest somewhat schematically some of the factors capable of such treatment. Taken on that basis it will be understood that Figs. 1 to 4 are intended to represent diflerent types of particles, under high magnification, to illustrate different characteristic later to be described. while Fig. 5 suggests a surface coat or layer of particles according to my invention with the paths of incident light indicated by broken and full lines meant to show long and short wave paths,

respectively. d

Fig. 6 is a graph indicating the degree of reflectance of a surface coat according to my invention plotted on reflectance as the ordinate and wave lengths in millimicrons as the abscissa, and

Fig. '7 is a similar graph plotted for reflectance of a surface painted with a blue paint tinted to match the paint in Fig. 6 as closely as possible.'

Fig, 1 is intended to show a dispersion of the colloidal pigment in a liquid, paint, enamel or lacquer or as a film of these, drawn schematically and at very great magnification. In this figure the dispersed pigments are represented by dots 20. The coarse dark or black particles are in silhouette, as suggested at It.

It will be noted that the concentration of colloidal particles is greater around the coarse ones This same result maybe attained mechanically by intimately mixing and squeezing together in a dry state, as in an edge runner or ball mill before dispersing in the liquid vehicle. This concentration of line particles I have attempted to suggest schematically in Fig. 2. a

Fig. 3 is intended'to show the type of dispersion in which the coarse black particles are produced by agglomerates of finer ones as at 30.

Fig. 4 shows the type where the relatively flne black or dark particles l0 are adsorbed on a coarse inert extender 40, such as talc.

Fig. 5 is intended to illustrate schematically the film coating, greatly magnified, and the long and short light waves with their paths incident to said film.

The light striking the black or dark particles is quenched and converted to heat, the light of long wave length passing through the colloidal particles, and the light of short wave len th being scattered by these particles indicated by the arrows. This short wave length light passes out through the film as a hazy blue 01 the same spectrum as the blue of the sky.

It will therefore be understood, according to my present invention, that whether the materials l0 and 20 were applied separately or whether they were mixed and applied together, the general effect would be largely the same. This effect is that before described approximating the blue or blue-gray tints of the Tyndall eiIect, and it will be understood that with the varying character of the light these eflects will be changeable with changes in the intensity of the sunlight or the presence of greater or less amounts of moisture, smoke, dust or other impurities in the air.

The variation of these effects is one of the important accomplishments of my invention as an airplane having its surfaces treated in accordance with my invention will present a varying appearance from time to time, sometimes from moment to moment.

Having explained the basis of my invention and the factorse involved, I will now give a few illustrative examples of formulae which will sug gest to those skilled in the art all the infinite number of variants which may be made to get a paint, enamel or lacquer for forming a surface fllm or coating.

As an example of my invention:

1. I disperse titanium oxide in polymerized linseed oil and coal tar hydrocarbons. The proportion of these ingredients is as follows: Titanium oxide 50 parts, polymerized linseed oil 25 parts, xylene 50 parts, by weight. These ingredients are then centrifuged to eliminate all but the finest colloidal particles. Into this mixture is then stirred a small quantity of mineral black of sizes between 1 and 50 microns. The amount added is that required to get the desired depth of shade. Calcium carbonate in the amount of 30 parts may be added to flat the mixture. I may also use talc, silica or sand as fillers or extenders and these will also tend to fiat the finished surface of a film or coating applied to an object.

II. As another example, I disperse zinc oxide in an oil modified neutral resin varnish, such as glycerol phthalate 50 parts, hydrocarbon thinner 25 parts for 50 parts of the zinc oxide. Instead of glycerol phthalate I may use a lacquer such as a cellulose ester.

A small amount of lampblack may be worked dry in an edge runner or, a mixer with talc, sand or other inert extender until the black color is thoroughly developed on the talc surface. This is then added to the zinc oxide-glycerol phthalate dispersion.

Products in accordance with my invention may be brushed or sprayed on to any surface or surfaces of the object, the visibility of which is to be reduced, and if the object is subject to unusual conditions of heat, cold, moisture or even of leaking oil and chemicals, a transparent protective coat may be used over the surface to which the dimming effect has been applied.

Referring to the two graphs of Figs. 6 and 7, these are altered as a basic of comparison between the efiects produced by my surface coat as plotted in Fig. 6, which is substantially identical with that of blue sky at zenith, and that of a surface simply painted with a blue paint based on prussian blue as a pigment and supposed to match the blue sky at zenith, as plotted in Fig. 7.

On these graphs the percentage of reflectance is the ordinate and wave lengths in millimicrons is the abscissa.

The practical difference is apparent when the underside of an airplan is treated according to my invention and a like surface simply painted with a blue paint.

In the first instance (Fig. 6) the airplane surface viewed against a sky is variantly and diminishingly reduced in visibility according to altitude. In the instance of the use of blue paint (Fig. '7) which never really matches the sky, its visibility is constantly definite.

A. pigment color blue, interpreted in simple paint pigment such as prussian blue is very definitely visible and recognizable under almost any usual condition or set of conditions. The blue or blue-green effects produced by the various transitions of solar light is not a color but a dimming by dispersion in which the apparent color is largely incidental. In accordance with my invention I can produce this artificially on a surface.

It wfll be noted that in the graph of Fig. 6

the percentage o1 intensity in that instance is reflectance from a sky based on zenith. In the graph of Fig. 7 the percentage is based on refiectance from a surface simply painted a blue color.

The wave length or spectrum lay-oil? in the abscissa or horizontal scale is tabulated in millimicrons because these represent factors more relatedto the light eflects of the problem than in the usual analyses based on mere color.

What I therefore claim and desire to secure by Letters Patent is:

1. A paint, enamel or pigmented lacquer for reducing the visibility of an object when seen against the sky, water or distant backgroud, comprising pigments in diflerent degrees of dispersion, including a white pigment opaque to blue light, dispersed to a fineness below the wave length of visible light, and a dark, light-absorptive pigment of dimension greater than the wave length of visible light surrounded by a continuous phase of highly dispersed fine particles opaque to the bluish light.

2. A paint, enamel or pigmented lacquer as in claim 1, the dispersion being in a drying oil and a' hydrocarbon solvent.

3. A .paint, enamel or pigmented lacquer as in claim 1, the dispersion being in a drying oil and a hydrocarbon solvent and tral resin soluble therein.

4. A paint, enamel or pigmented lacquer as in claim 1, the dispersion being in a-vehicle consisting of an oil soluble glycerol phthalate resin dispersed in a suitable solvent.

5. A paint, enamel or pigmented lacquer as in claim 1, the dispersion being in a vehicle consisting of a cellulose ester dissolved in a. solvent.

6. A paint, enamel, or pigmented lacquer as in claim 1, and an amount of relatively coarse, colorless mineral substance productive of a fiat surface.

-'7. A paint, enamel or pigmented lacquer for reducing the visibility of an object when seen against the sky, water or distant background, comprising pigments in different degrees of dispersion, including a whitish pigment opaque to blue light, dispersed to a fineness below the wave length of visible light, and a dark, light-absorptive pigment of dimension greater than the wave length of visible light surrounded by a continuous phase of the fine whitish pigment-particles, and an extender comprising relatively coarse substantially colorless particles surrounded by said dark pigment.

8. A paint, enamel or pigmented lacquer as in claim 7, in which the extender is talc.

9. A paint, enamel or pigmented lacquer as in claim 7, in which the extender is silica.

SAMUEL CABO'I.

a substantially neu- 

